This invention relates to methods for controlling controllable clutches in the drive train between a front axle and a rear axle of a motor vehicle having four-wheel drive, in which each of the wheels has a wheel-speed sensor providing an output signal which is supplied to an evaluation unit with a data memory.
German Patent No. 37 21 626 C2 discloses a clutch control arrangement of this type in which the wheel speed is determined by wheel-speed sensors. In that clutch control arrangement an actuator acts on the clutch in such a way as to reduce the torque transmitted if the time derivative of a reduction in wheel speed exceeds a predetermined value. A clutch control system of this kind thus improves only the braking behavior and does not take into account exceptional situations, for example, spinning of the wheels. Moreover, clutch control distortions which can occur during cornering because of the rotational speed differences produced in the clutch, due to different front and rear wheel speed differences during cornering or due to different wheel radii, are not taken into account.
German Offenlegungsschrift No. 43 27 507 A1 discloses a clutch arrangement for heavy vehicles used on difficult terrain. In this case, however, lock-up clutches are provided for each of the front and rear axle differentials and a selectable clutch is included between the front and rear axles.
German Patent No. 35 04 455 C3 discloses a drive control arrangement for a road vehicle which includes three locking differentials. In this control arrangement, the only control positions provided are "locking differential locked" and "locking differential unlocked". There are no intermediate positions provided for the purpose of varying the torque transmitted by the locking differentials.